kaluki, we had similar here (I say "had", because DH has been really working to address this and it does seem to have lessened quite a bit).
DSD was very much favoured by DH. They are more alike, in some ways, then DH and DSS. I also know that when the DSCs mum was alive, she was very close to DSS as her 'baby', and in part I am told to compensate for the closeness (she felt) between DH and DSD - so that their family was almost two teams - DH/DSD, and mum/DSS. This meant that they had just years and years of kind of excluding DSS, and then that was all exaggerated by the way that DSD was 'spousified', so that she was incredibly bossy and domineering to DSS, who was really infantalised.
When I met them, DSS was simultaneously very close to DSD, and very, very resentful and angry about her dominant role, and sadly, I think they both rather believed that DH loved DSS more.
DH caught on to this eventually, but only as DSS became more confident and assertive...DH was gutted by it, and like I say, he's now much more careful to respect DSS.
I found it much easier to get along with DSS, partly because we are simply more temperamentally alike, but also because he was almost grateful when I came along, I think it restored a bit of balance that was missing - he wasn't so outnumbered anymore! Now he is a stroppy teenager, but I think I am still his biggest advocate in the household, because I will call DH out when I see him treating them unfairly.
On the other hand, it definitely made things tougher with DSD, because not only was she now having to share daddy, and losing her role as 'lady of the house', but she was also not being allowed to lord it over DSS quite so much.